1. Field of the Invention
Exemplary embodiments of the present invention relate to a semiconductor design technology, and more particularly, to a semiconductor memory device that performs a program operation and a read operation and an operating method of the semiconductor memory device.
2. Discussion of Related Art
In general, a semiconductor memory device, such as a flash memory device, performs a program/erase operation to store data in a memory cell, and performs a read operation to output the stored data.
The program operation causes an increase in a threshold voltage of the memory cell. When a high voltage is supplied to a control gate during the program operation, electrons are injected into a floating gate by Fowler-Nordheim (FN) tunneling, and the threshold voltage of the memory cell increases. Repeated program/erase operations on the memory cell may cause the electrons injected into the floating gate to be trapped in a tunnel oxide layer. For this reason, an increase in cycling number may cause an increase in the programming speed of memory cells.
It may be seen from FIG. 1 that the greater the cycling number, the wider the right portion of Nth distribution.
Meanwhile, particularly in a NAND flash memory among semiconductor memory devices, stored data should be maintained for a long time (e.g., 10 years or more) even when powers are turned off, which is referred to as reliability. As the flash memory is highly integrated, the size of a memory cell decreases, and an interval between respective distributions becomes very narrow. The narrow interval requires accurate adjustment of the number of the electrons stored in a floating gate, and prevention of electron loss over time for the reliability. However, since the size of the memory cell decreases and the number of the electrons stored in the memory cell is reduced, the loss of the electrons has a strong influence on distributions. The electrons denote negative charge, and the loss of the electrons moves distributions to the left.
It may be seen from FIG. 1 that the greater the retention number, the wider the left portion of (N+1)th distribution.
For this reason, there may be a concern regarding an overlap between the left side cells of the reference distribution (e.g., (N+1)th distribution) and the right side cells of the distribution (e.g., (N)th distribution) immediately below the reference distribution.